Recently, near-field light generating devices are vigorously developed which convert propagating light into near-field light. Application of the near-field light generating devices to an optical circuit, a recording head, and a recording device is actively proposed. In the field of optical recording, miniaturization of an optical spot is advanced for a higher recording density. In view of this, use of near-field light is proposed. In the use of near-field light, a high intensity of near-field light is required for a high S/N ratio. For this reason, a near-field light generating device is used which converts propagating light into near-field light by, particularly, a surface plasmon polariton technology.
For optically-assisted magnetic recording, it is necessary to take into consideration relative positions of a near-field light generating device, a magnetic pole, and a reproducing element.
For example, in an optically-assisted magnetic recording head disclosed in Patent Literature 1, a metal film having an aperture at an output terminal of a semiconductor laser is formed so that near-field light may be generated, by the metal film, through the use of surface plasmon polariton enhancement. FIG. 18 is a view illustrating an arrangement of the metal film disclosed in Patent Literature 1. As shown in FIG. 18, the metal film 95 has a cross-like aperture 96. Near-field light is generated between vertexes that are adjacent in a polarization direction which vertexes appear as a result of formation of the cross-like aperture.
In an optically-assisted magnetic recording head disclosed in Patent Literature 2, electrically conductive scatterers for generating near-field light are disposed within a magnetic field generating coil. An internal width of the coil is equal to or smaller than a wavelength of light that enters the coil, and an outside diameter of the coil is larger than a spot diameter of light that enters the coil. The arrangement shown in FIG. 19 is disclosed as an exemplary arrangement of a head including a coil and scatterers.
FIG. 19 is a diagram illustrating an arrangement of the head disclosed in Patent Literature 2 which includes a coil and scatterers. As shown in FIG. 19, two metal scatterers 92 having a triangular shape are disposed so as to be in contact with a magnetic field generating coil 93.
In a case where the scatterers 92 are irradiated with incident light having a polarization direction as indicated by the arrow of FIG. 19, near-field light is generated in the scatterers 92. Meanwhile, a magnetic field is generated at a central part of the magnetic field generating coil 93 by passing an electric current through the magnetic field generating coil 93. Accordingly, in a case where the scatterers 92 are formed at the center of the magnetic field generating coil 93, a magnetic field and near-field light can be generated at the same location.
Meanwhile, Non-patent Literature 1 teaches that in a case where light polarized in a width direction of a V-shaped groove enters a V-shaped near-field light generating device, generated surface plasmon polaritons converge at a tip of the V-shaped groove. This is described below with reference to (a) through (d) of FIG. 20.
(a) of FIG. 20 is a perspective view illustrating an arrangement of a near-field light generating device disclosed in Non-patent Literature 1. (b) of FIG. 20 is a cross-sectional view of the near-field light generating device of (a) of FIG. 20, in which view a cross section parallel to an X-Y plane is illustrated. (c) of FIG. 20 is a cross-sectional view of the near-field light generating device of (a) of FIG. 20, in which view a cross section parallel to an Y-Z plane and propagation of surface plasmon polaritons are illustrated. (d) of FIG. 20 is a diagram illustrating the propagation illustrated in (c) of FIG. 20.
As illustrated in (a) of FIG. 20, X, Y, and Z axes are assumed. A near-field light generating device 100 consists of a metallic member 101 and a dielectric member 102. The metallic member 101 has a groove whose cross-section parallel with an X-Y plane is a V-shape. The dielectric member 102 is provided in the groove.
As illustrated in (b) of FIG. 20, a width, in a direction of an X-axis, of the groove formed in the metallic member 101 (i.e., a width of the dielectric member 102) becomes narrower from a plus direction of a Y-axis to a minus direction of the Y-axis. The narrower the width in the direction of the X-axis, the larger the effective refractive index for surface plasmon polaritons excited in a case where light polarized in the direction of the X-axis enters the near-field light generating device 100. In this case, a track of the surface plasmon polaritons propagating through the groove in the metallic member 101 is indicated by the arrow A in (c) of FIG. 20. That is, the surface plasmon polaritons change their propagation direction toward a tip of the groove.
If incident light travels from a medium having a small refractive index to a medium having a large refractive index, θ4<θ3 is satisfied by Snell's law, as illustrated in (d) of FIG. 20. Since a groove of a V-shaped near-field light generating device such as the near-field light generating device 100 is considered to be a group of layers in which a refractive index gradually changes, the surface plasmon polaritons propagating through the groove in the metallic member 101 converge at the tip of the V-shape of the groove.
As indicated by the dashed line in (d) of FIG. 20, usually, light (surface plasmon polaritons) is reflected on an interface between two media which are different in refractive index from each other. However, if a difference between the refractive indexes of the two media is very small, a reflectance is very small. That is, decreasing an angle of an opening of the V-shape of the groove makes it possible to decrease a change in effective refractive index. As a result, reflection of the light can be suppressed in the V-shaped near-field light generating device 100 so that the surface plasmon polaritons may be converged at a Z-axis (i.e., at the tip of the groove).
Further, Non-patent Literatures 2 and 3 teach that there exists a coupling mode of surface plasmon polaritons propagating along vertexes of two metal wedges facing each other.
FIG. 21 is a perspective view illustrating an arrangement of the near-field light generating device disclosed in Non-patent Literature 2. As shown in FIG. 21, the near-field light generating device 200 includes a metal wedge 200a and a metal wedge 200b, each of which is made of a metallic material. The metal wedge 200a and the metal wedge 200b are disposed so as to face each other and so that their respective vertexes are away from each other. Surface plasmon polaritons propagate in a direction (X-axis direction of FIG. 21) in which the vertex of the metal wedge 200a and the vertex of the metal wedge 200b extend.
Which of the phenomenon described in Non-patent Literature 1 and the phenomenon described in Non-patent Literatures 2 and 3 occurs depends on a vertex of a V-shaped near-field light generating device (or metal wedges), a direction in which surface plasmon polaritons enter a flection, and inner-interface distance at the location of the flection and in the vicinity of the flection.